1. Field of the Invention
Cylindrical prestressed concrete pipe is widely used today for conveying pressure fluids. Most such concrete pipe includes a primary cylindrical metal core which is either completely embedded in the concrete or disposed adjacent the outer surface of the concrete. The primary core is subsequently prestressed by circumferentially wrapping steel wire at a predetermined tension throughout the entire length thereof. The prestressed core is then covered with a concrete coating to protect the wire wrapping. It will be appreciated that concrete pipe having such a primary core differs substantially from standard concrete pipe of the type utilized in storm and sanitary sewers, both in structure and in application.
2. Description of the Prior Art
The traditional method of forming the latter of the above mentioned primary cores involves the step of initially orienting the longitudinal axis of the metal cylinder in a horizontal direction. The metal cylinder is then rapidly rotated about its longitudinal axis and concrete is applied to the interior surface thereof. Centrifugal action causes the concrete to accumulate and, thereby, bind to the inner wall of the rotating steel cylinder. The concrete is then allowed to cure, and the steel cylinder containing the inner lining of concrete is removed from the manipulating apparatus.
The traditional method of forming the former of the above mentioned primary cores involves the step of initially disposing the steel cylinder vertically between cooperating inner and outer mold forms. Concrete is then introduced between the cooperating mold forms and compacted by vibrating the inner and outer mold forms. After the concrete has been allowed to cure, the inner and outer mold sections are removed, permitting removal of the primary core.
The packerhead method of forming conventional or standard concrete pipe has been known for some time and is widely accepted as the least expensive and most efficient method of forming such concrete pipe. Generally, a machine for forming pipe by this method includes an outer vertically disposed jacket for forming the outer surface of a pipe section and an inner vertically reciprocable rotary packerhead. The packerhead is concentrically disposed with respect to the jacket for forming the interior surface of the pipe section. The completed pipe section may typically be formed to include a female end and a male end on opposite ends thereof.
The conventional steps in the process for forming standard pipe are as follows. The jacket is disposed vertically with the female end facing downwardly. The rotary packerhead is then lowered concentrically within the jacket to the bottom thereof. The packerhead includes a rotating cylindrically shaped base member having an outer diameter equal to the inside diameter of the completed pipe section, a rotary pipe forming assembly from which the base member depends, and a plurality of radially spaced trowels or rollers for forming and shaping the interior surface of the pipe section. A circular plate having a plurality of vertically disposed concrete slingers thereon can be secured to the packerhead above the plurality of trowels or rollers for initially forming the pipe. A shaft supports and rotates the entire packerhead assembly.
Concrete is introduced into the interior of the jacket through the upper end thereof. The packerhead is rotated to form the female end of the pipe. Once the female end of the pipe is formed, the rotating packerhead is raised while concrete is introduced into the jacket to form the cylindrical body of the pipe. When the rotating packerhead reaches the uppermost terminus of travel, and prior to the time the concrete has cured, an additional pass of the packerhead vertically throughout the jacket can be accomplished to effectively provide a smooth finish to the interior surface of the formed pipe. During pipe formation and thereafter, the jacket may be vibrated to assist in causing the concrete to compact and eliminate any voids which might occur in the pipe wall. Thereafter, the packerhead is removed and the jacket with the completed pipe therein is moved for curing, whereupon a new jacket is provided and the process repeated.
U.S. Pat. No. 3,276,091 to Pausch discloses an apparatus for forming concrete pipes including a roller head and troweling cylinder having a downwardly depending cylindrical skirt. A head plate is bolted across the upper end of the troweling cylinder. An annular array of symmetrically and circumferentially spaced eccentrically adjustable roller packer units is disposed to extend upwardly from the head plate. An aggregate spreader plate is disposed above the roller packer units. A plurality of aggregate slingers disposed on the spreader plate is adapted to propel aggregate into the paths of the roller packer units.
U.S. Pat. No. 3,733,163 to Hermann discloses an improved wear surface for long bottoms employed in concrete pipe making machines utilizing the packerhead method of pipe formation. The long bottom of the packerhead includes a plurality of wear segments disposed about the upper circumferential outer surface thereof. U.S. Pat. No. 3,829,268 to Gill discloses a packerhead concrete pipe machine operable to make a cylindrical concrete pipe in an upright mold. The machine has a triangular frame assembly formed with three upright frame members, each affixed to each other by suitable cross braces. A rotatable turntable has a center hole for accommodating one of the frame members. The turntable carries three molds and is sequentially rotated to move the molds in longitudinal alignment with the packerhead and a cylindrical core having vibrating units.
U.S. Pat. No. 3,922,133 to Crawford et al. discloses a packerhead/vibration system for a concrete pipe machine. An overfill and compaction ring assembly is mounted on the top of the pipe mold, enabling complete formation of the concrete pipe in one stage. The method of vibratory concrete pipe manufacture includes the steps of overfilling the pipe mold in the pipe-forming process, vibrating the concrete forming the pipe, and compressing the overfilled concrete into a space created by densification of the pipe material caused by vibration.
Other related concrete pipe forming systems are disclosed in U.S. Pat. Nos. 2,178,015 to Brunetti, 2,751,657 to Holston, 2,966,766 to Ronaldson et al., 3,096,556 to Woods, 3,262,175 to Gourlie et al., 3,551,968 to Fosse et al., 3,649,727 to Gauger, 4,197,074 to Christian, 4,334,848 to Gross et al., 4,336,013 to Hand, and 4,340,553 to Fosse.